3,515 research outputs found
Dynamics of sliding drops on superhydrophobic surfaces
We use a free energy lattice Boltzmann approach to investigate numerically
the dynamics of drops moving across superhydrophobic surfaces. The surfaces
comprise a regular array of posts small compared to the drop size. For drops
suspended on the posts the velocity increases as the number of posts decreases.
We show that this is because the velocity is primarily determined by the
contact angle which, in turn, depends on the area covered by posts. Collapsed
drops, which fill the interstices between the posts, behave in a very different
way. The posts now impede the drop behaviour and the velocity falls as their
density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let
Rheology of cholesteric blue phases
Blue phases of cholesteric liquid crystals offer a spectacular example of
naturally occurring disclination line networks. Here we numerically solve the
hydrodynamic equations of motion to investigate the response of three types of
blue phases to an imposed Poiseuille flow. We show that shear forces bend and
twist and can unzip the disclination lines. Under gentle forcing the network
opposes the flow and the apparent viscosity is significantly higher than that
of an isotropic liquid. With increased forcing we find strong shear thinning
corresponding to the disruption of the defect network. As the viscosity starts
to drop, the imposed flow sets the network into motion. Disclinations break-up
and re-form with their neighbours in the flow direction. This gives rise to
oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure
Quantum non-malleability and authentication
In encryption, non-malleability is a highly desirable property: it ensures
that adversaries cannot manipulate the plaintext by acting on the ciphertext.
Ambainis, Bouda and Winter gave a definition of non-malleability for the
encryption of quantum data. In this work, we show that this definition is too
weak, as it allows adversaries to "inject" plaintexts of their choice into the
ciphertext. We give a new definition of quantum non-malleability which resolves
this problem. Our definition is expressed in terms of entropic quantities,
considers stronger adversaries, and does not assume secrecy. Rather, we prove
that quantum non-malleability implies secrecy; this is in stark contrast to the
classical setting, where the two properties are completely independent. For
unitary schemes, our notion of non-malleability is equivalent to encryption
with a two-design (and hence also to the definition of Ambainis et al.). Our
techniques also yield new results regarding the closely-related task of quantum
authentication. We show that "total authentication" (a notion recently proposed
by Garg, Yuen and Zhandry) can be satisfied with two-designs, a significant
improvement over the eight-design construction of Garg et al. We also show
that, under a mild adaptation of the rejection procedure, both total
authentication and our notion of non-malleability yield quantum authentication
as defined by Dupuis, Nielsen and Salvail.Comment: 20+13 pages, one figure. v2: published version plus extra material.
v3: references added and update
Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces
We report experiments investigating the behaviour of micron-scale fluid
droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes.
The final droplet shape depends on the droplet size relative to that of the
stripes. In particular when the droplet radius is of the same order as the
stripe width, the final shape is determined by the dynamic evolution of the
drop and shows a sensitive dependence on the initial droplet position and
velocity. Numerical solutions of the dynamical equations of motion of the drop
provide a close quantitative match to the experimental results. This proves
helpful in interpreting the data and allows for accurate prediction of fluid
droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui
Control of drop positioning using chemical patterning
We explore how chemical patterning on surfaces can be used to control drop
wetting. Both numerical and experimental results are presented to show how the
dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic
grid. The grid proves a successful way of confining drops and we show that it
can be used to alleviate {\it mottle}, a degradation in image quality which
results from uneven drop coalescence due to randomness in the positions of the
drops within the jetted array.Comment: 3 pages, 4 figure
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Collective modes in a system with two spin-density waves: the `Ribault' phase of quasi-one-dimensional organic conductors
We study the long-wavelength collective modes in the magnetic-field-induced
spin-density-wave (FISDW) phases experimentally observed in organic conductors
of the Bechgaard salts family, focusing on phases that exhibit a sign reversal
of the quantum Hall effect (Ribault anomaly). We have recently proposed that
two SDW's coexist in the Ribault phase, as a result of Umklapp processes. When
the latter are strong enough, the two SDW's become circularly polarized
(helicoidal SDW's). In this paper, we study the collective modes which result
from the presence of two SDW's. We find two Goldstone modes, an out-of-phase
sliding mode and an in-phase spin-wave mode, and two gapped modes. The sliding
Goldstone mode carries only a fraction of the total optical spectral weight,
which is determined by the ratio of the amplitude of the two SDW's. In the
helicoidal phase, all the spectral weight is pushed up above the SDW gap. We
also point out similarities with phase modes in two-band or bilayer
superconductors. We expect our conclusions to hold for generic two-SDW systems.Comment: Revised version, 25 pages, RevTex, 7 figure
Unforgeable Quantum Encryption
We study the problem of encrypting and authenticating quantum data in the
presence of adversaries making adaptive chosen plaintext and chosen ciphertext
queries. Classically, security games use string copying and comparison to
detect adversarial cheating in such scenarios. Quantumly, this approach would
violate no-cloning. We develop new techniques to overcome this problem: we use
entanglement to detect cheating, and rely on recent results for characterizing
quantum encryption schemes. We give definitions for (i.) ciphertext
unforgeability , (ii.) indistinguishability under adaptive chosen-ciphertext
attack, and (iii.) authenticated encryption. The restriction of each definition
to the classical setting is at least as strong as the corresponding classical
notion: (i) implies INT-CTXT, (ii) implies IND-CCA2, and (iii) implies AE. All
of our new notions also imply QIND-CPA privacy. Combining one-time
authentication and classical pseudorandomness, we construct schemes for each of
these new quantum security notions, and provide several separation examples.
Along the way, we also give a new definition of one-time quantum authentication
which, unlike all previous approaches, authenticates ciphertexts rather than
plaintexts.Comment: 22+2 pages, 1 figure. v3: error in the definition of QIND-CCA2 fixed,
some proofs related to QIND-CCA2 clarifie
Ordering dynamics of blue phases entails kinetic stabilization of amorphous networks
The cubic blue phases of liquid crystals are fascinating and technologically
promising examples of hierarchically structured soft materials, comprising
ordered networks of defect lines (disclinations) within a liquid crystalline
matrix. We present the first large-scale simulations of their domain growth,
starting from a blue phase nucleus within a supercooled isotropic or
cholesteric background. The nucleated phase is thermodynamically stable; one
expects its slow orderly growth, creating a bulk cubic. Instead, we find that
the strong propensity to form disclinations drives the rapid disorderly growth
of a metastable amorphous defect network. During this process the original
nucleus is destroyed; re-emergence of the stable phase may therefore require a
second nucleation step. Our findings suggest that blue phases exhibit
hierarchical behavior in their ordering dynamics, to match that in their
structure.Comment: 11 pages, 5 figures, 2 supplementary figures, 2 supplementary tables,
accepted by PNA
Decoupling with unitary approximate two-designs
Consider a bipartite system, of which one subsystem, A, undergoes a physical
evolution separated from the other subsystem, R. One may ask under which
conditions this evolution destroys all initial correlations between the
subsystems A and R, i.e. decouples the subsystems. A quantitative answer to
this question is provided by decoupling theorems, which have been developed
recently in the area of quantum information theory. This paper builds on
preceding work, which shows that decoupling is achieved if the evolution on A
consists of a typical unitary, chosen with respect to the Haar measure,
followed by a process that adds sufficient decoherence. Here, we prove a
generalized decoupling theorem for the case where the unitary is chosen from an
approximate two-design. A main implication of this result is that decoupling is
physical, in the sense that it occurs already for short sequences of random
two-body interactions, which can be modeled as efficient circuits. Our
decoupling result is independent of the dimension of the R system, which shows
that approximate 2-designs are appropriate for decoupling even if the dimension
of this system is large.Comment: Published versio
- …